This study is concerned with the elimination of inactivation of blocking antibody. We are examining the specific in vivo and in vitro effects of a rabbit anti-mouse plasma cell sera (APS) on several murine tumors. The in vitro studies examine the mitogenic response of lymphoid cells after treatment with APS. The in vivo studies are concerned with the effect of APS in mice with rapidly growing enhanced tumors. Two basic treatment regimens are utilized: the tumors are surgically excised and the animals treated with APS; the tumors are treated after they have reached a certain growth size with APS to determine the effectiveness of such treatment on an actively growing tumor. A second phase of this study concerns the preparation and in vitro testing of a rabbit anti-human plasma(HuAPS) cell sera. The rabbit anti-human plasma cell sera is studied by standard immunological techniques including the mitogenic response of human lymphocytes and their ability to form plaques after treatment with HuAPS. Since it has been postulated that blocking antibody may inhibit the normal in vivo cellular response of lymphoid cells against tumor cells, a specific humoral immunosuppressant may be able to negate or reduce the effect of blocking antibody.